The present invention relates to a standard component and to a calibration method using it. Especially, the invention relates to a standard component for an electron beam and to a calibration method for it.
Heretofore, when a microscope or other similar instrument such as an electron microscope or AFM (atomic force microscope) is adjusted, a standard sample for calibration has been used. The standard sample for calibration is a member on which a pattern or marks having a previously known size or length are formed. When the instrument is adjusted, an image of the pattern or marks is acquired at a certain magnification. The instrument is adjusted such that the pattern or marks are displayed in a given size on the acquired image.
Heretofore, a member made of a semiconductor substrate several millimeters to several centimeters square on which a one-dimensional diffraction grating pattern is formed has been used as a standard component for an electron microscope. The whole of a surface on which the pattern of the standard component is formed is shown in FIG. 12A. FIG. 12B is an enlarged view of a part of the surface shown in FIG. 12A. Grooves in the one-dimensional diffraction grating pattern formed on the front surface of the substrate are indicated by numeral 121. Since the accuracy of calibration is determined depending on the pitch dimension of the patterned grooves in the one-dimensional diffraction gratings, it is necessary to form the patterned grooves accurately. However, it is difficult to form grooves with fine pitch dimensions with high accuracy. Under the present situation, the accuracy of the pitch dimensions of patterned grooves that can be formed is approximately 1 nm.
JP-A-2004-251682 discloses a standard component having a semiconductor substrate on which plural diffraction grating units are formed. The diffraction grating units have one-dimensional diffraction gratings in which subgratings are formed. Cross marks for alignment are formed around the diffraction grating units. In the invention described in the above-cited JP-A-2004-251682, diffraction grating units used for calibration are selected, using the cross marks formed around the array of one-dimensional diffraction grating pattern units and the cross marks arranged between the one-dimensional diffraction grating pattern units. In particular, the standard component placed on a sample stage is moved into a primary electron beam irradiation position by stage movement. The cross marks formed around the diffraction grating units are detected. Since the coordinates of the mark positions on the stage and the coordinates of the diffraction grating units on the stage to which the component should be moved are already known, the amount of movement of the stage from any mark position and the movement target can be calculated. Consequently, the stage is made to move the calculated amount of movement. The one-dimensional diffraction grating unit forming a target position is moved into the primary electron beam irradiation position. The microscope is configured using a microscope image of the diffraction grating unit in a given position, the image being derived as described above.
Where a microscope or other similar instrument is calibrated using a standard component made up of plural diffraction grating units such as the standard component disclosed in the above-cited JP-A-2004-251682, it is necessary to move the diffraction grating unit used for calibration into the field of view of the microscope image. In this case, the following problems take place.
First, there is the problem that it is impossible to check whether or not the diffraction grating unit moved into the field of view of the microscope image by stage movement is a correct diffraction grating unit. In the standard component disclosed in the above-cited JP-A-2004-251682, diffraction grating units with the exactly identical pattern are formed up and down and right and left. It is substantially impossible for an unskilled operator to visually discriminate between individual diffraction grating units on the image. It is difficult even for a skilled operator to perform this operation.
Furthermore, in a normal microscope or other similar instrument, movement of a desired diffraction grating unit into the field of view of a microscope image is controlled using only information about the coordinates of the stage. Accordingly, if the coordinates of the position used in controlling the stage are in error, it is impossible to move the intended diffraction grating unit into the field of view of the microscope image. In addition, because of the limitations of mechanical control, the accuracy of the stage position control is on the micrometer order at best. Consequently, in order that the intended diffraction grating unit be placed within the field of view, it is necessary to provide control for modifying the electron beam irradiation position, in addition to stage movement. Accordingly, where calibration is performed using a standard component made up of plural diffraction grating units, diffraction grating units are selected by making use of only stage movement consciously of involvement of uncertainty. Alternatively, a skilled operator moves a desired diffraction grating unit into the field of view while relying on his intuition.
Secondly, because information used in controlling the movement of the stage is only information about the coordinates of cross marks and the target position, if any one of the cross marks cannot be detected because of contamination or defectiveness, it is impossible or difficult to move the stage itself.